2007 Vauxhall Astra estate tailgate won't unlock?

My 2007 Vauxhall Astra tailgate won't unlock. When I press the unlock button on the central locking remote, the 4 doors unlock ok, but the tailgate remains locked.
Anything obvious I could check please?
The car's booked in for a routine sevice after Xmas, so it will (hopefully) get sorted them, so it's not a major problem, but it's a pain having to load & unload stuff by leaning over the rear seats into the boot.
Thanks in advance.